Collectorless Flotation

For collectorless flotation, the formation of a metal-deficient sulfide layer on the particle surface must generally be targeted. Formation of such a layer may be accelerated and spatially accentuated by galvanic interaction. When considering the flotation of sulfide minerals in the absence of collector reagents, in the context of electrochemical mineral interaction, three factors can induce floatability. First of all, floatability can be natural, i.e. due to the crystal structure and chemical bonding of a mineral. Examples of such minerals are molybdenite, stibnite, and the arsenic sulfides realgar and orpiment. Secondly, collectorless flotation can be self-induced, i.e. under the right pulp oxidative potentials, surface products will form which induce hydrophobicity. Examples of this are pyrrhotite and chalcopyrite. Rao and Finch found that pyrite/sphalerite selectivity could be enhanced by first recovering the pyrite which is naturally floatable, due to a chemically formed sulfur layer, in the absence of a collector. A third cause of floatability is mineral size. As minerals decrease in size, their recovery into flotation froths increases due to entrain-ment, rather than selective attachment to froth bubbles. A good example of this is galena flotation from complex ores. At one mine site, carrying out sequential copper-lead-zinc flotation, about three-quarters of lead recovery into the copper concentrate was found to be made up of galena particles smaller than 10 |im. Since this effect is physical rather than chemical, it can only be significantly affected by a change in physical parameters, e.g. froth lamellae thickness and particle size.

In the collectorless flotation of pyrite-chalco-pyrite-quartz mixtures, Johnson found a dependence of flotation behaviour on the pyrite/chalcopyrite surface area ratio, which would be consistent with electrochemical interaction. In this work it was however shown as well that copper solubilization was not enhanced in the presence of pyrite, but rather reduced; this points to copper deposition on pyrite - in other words, to activation rather than direct electrochemical interaction. Interestingly, interaction was reduced in the presence of quartz, due to adsorption of metal ions onto the quartz surfaces. In this respect, adsorption studies on other gangue minerals showed that such scavenging of potentially activating ions from solution may be substantial.

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